Design and Control of Stacked-Column Distillation Systems

The separation of close-boiling, low-relative volatility components by distillation requires many trays and high reflux ratios (high energy consumption). Temperature-sensitive components often limit temperatures, so vacuum conditions must be used in which pressure drop through trays or packing becomes very important. More trays produce higher base pressures and higher base temperatures for a given pressure in the reflux drum. Therefore, it may be impossible to achieve the desired separation in single column if the maximum temperature limits the number of trays. In this situation, multiple columns must be used to stay within the feasible number of trays per vessel. Each column has its own condenser and reboiler. The bottoms from the upper column is fed to the top of the lower column, and the overhead vapor from the lower column is condensed, pumped to a higher pressure, and fed into the base of the upper column. This stacked column configuration has high energy and capital cost, but it may be the only feasible way to achieve the desired separation and not exceed the maximum temperature limitation. This paper explores the design and control of this complex process.